Generally, exhaust gases issuing from various conbustion devices contain NO.sub.x, occasionally in conjunction with other gaseous components such as sulfur oxides (hereinafter referred to as SO.sub.x). The NO.sub.x constitutes the principal cause for the so-called photochemical smog which has posed as a serious social problem. From the standpoint of prevention of air pollution, it is necessary that such an exhaust gas should be released into the atmosphere after it has been freed from NO.sub.x.
For the removal of NO.sub.x from an exhaust gas containing NO.sub.x, there has been proposed a method which includes contacting the exhaust gas with ammonia in the presence of a catalyst to react the NO.sub.x with ammonia and consequently converting the NO.sub.x into N.sub.2. The catalysts which have heretofore been found to be usable in this method include those obtained by impregnating an alumina carrier with oxides of active metals such as iron and copper, for example. The catalyst of this type, however, has the disadvantage that it fails to retain its catalytic activity stably for any appreciable length of time where the exhaust gas subjected to the treatment happens to contain SO.sub.x (for example, SO.sub.2 and SO.sub.3). Since Al.sub.2 O.sub.3 forming the carrier of this catalyst reacts with SO.sub.x and causes sulfation of Al.sub.2 O.sub.3 or adsorption of SO.sub.x onto Al.sub.2 O.sub.3, the points of activity on the catalyst are occluded eventually to the extent of degrading the catalytic activity. Further, in the case of a catalyst which contains Fe.sub.2 O.sub.3 as an active component, there is a possibility that SO.sub.x and Fe.sub.2 O.sub.3 will react to produce ferric sulfate and this reaction product will bring about degradation of the catalytic activity. For the removal of NO.sub.x from an exhaust gas containing the NO.sub.x by the catalytic reduction of the NO.sub.x with ammonia, therefore, the conventional catalysts described above cannot be called satisfactorily usable.